1. Field of the Invention
The invention relates to a variable capacitance element (variable capacitor) and, more particularly, to a variable capacitor that is fabricated with a Micro Electro Mechanical System (MEMS) technology, a matching circuit element that uses the variable capacitor, and a mobile terminal apparatus that uses the variable capacitor or the matching circuit element.
2. Description of the Related Art
A variable capacitor is an important component in an electric circuit, such as a variable frequency oscillator, a tuned amplifier, a phase shifter, and an impedance matching circuit. In recent years, the number of mobile terminal apparatuses in which a variable capacitor is mounted has been increasing. In the technical field of the mobile terminal apparatus, with an increase in the number of components mounted, and the like, for high performance, the need for miniaturization of components used is growing. To respond to the need, miniaturization of a variable capacitor is pursued utilizing a MEMS technology. The variable capacitor produced through the MEMS technology is advantageous in that a Q value may be increased because of a small loss as compared with a varactor diode that is mainly used currently, and development of the variable capacitor has been promoted.
A variable capacitor that is produced through the MEMS technology is, for example, described in Japanese Laid-open Patent Publication 2007-273932, in which the variable capacitor varies its capacitance by changing the distance between the opposite two electrodes.
FIG. 28 and FIG. 29 are cross-sectional views of an existing typical variable capacitor F1. The variable capacitor F1 includes a substrate 101, a fixed electrode 102, a movable electrode 104, a dielectric layer 105, and a pair of supports 106. The fixed electrode 102 is provided on the upper face (a face on the upper side in FIG. 28) of the substrate 101. The movable electrode 104 is provided above the fixed electrode 102 so as to be bridged with the pair of supports 106. The movable electrode 104 has a portion that faces the fixed electrode 102. The dielectric layer 105 is provided on the upper face of the fixed electrode 102 in order to prevent short circuit due to contact of the fixed electrode 102 with the movable electrode 104. The substrate 101 is made of a silicon material, and the fixed electrode 102 and the movable electrode 104 are respectively made of predetermined metal materials.
In the variable capacitor F1, a voltage applied between the fixed electrode 102 and the movable electrode 104, generates electrostatic attraction between them. Due to the electrostatic attraction, the movable electrode 104 is attracted toward the fixed electrode 102 to change the distance between the electrodes 102 and 104. This change in the distance varies a capacitance between the electrodes 102 and 104. Thus, by changing a voltage applied between the fixed electrode 102 and the movable electrode 104, it is possible to vary the capacitance.
Because of an extremely thin thickness of the dielectric layer 105, the capacitance of the variable capacitor F1 has a characteristic such that it is substantially inversely proportional to a distance d. The characteristic, however, differs in a region in which an influence on the capacitance value of the dielectric layer due to the distance d between the movable electrode 104 and the fixed electrode 102 cannot be ignored (for example, a region in which the movable electrode 104 is located in proximity to the fixed electrode 102).
Thus, the capacitance of the variable capacitor F1 is minimal in a state where the fixed electrode 102 is separated from the movable electrode 104 (a state where the distance d between the electrodes is maximal, see FIG. 28). On the contrary, the capacitance is maximal in a state where the fixed electrode 102 is in contact with the movable electrode 104 via the dielectric layer 105 (a state where the distance d between the electrodes is minimal, see FIG. 29).
FIG. 30 shows the variation of a capacitance of the variable capacitor F1 according to a driving voltage applied to the variable capacitor F1, which applied between the fixed electrode 102 and the movable electrode 104. The abscissa axis represents a driving voltage, and the ordinate axis represents a capacitance. As the driving voltage is increased, the capacitance abruptly increases and then attains a constant value (maximum capacitance) (see point P1). On the other hand, as the driving voltage is decreased, the capacitance abruptly decreases and then attains a constant value (minimum capacitance) (see point P2).
The characteristic of the capacitance in FIG. 30 varies such that it is inversely proportional to the distance d between the electrodes as described above. The point P1 is a point at which the fixed electrode 102 contacts the movable electrode 104 via the dielectric layer 105. The point P2 is a point at which electrostatic attraction between the fixed electrode 102 and the movable electrode 104 is lost. Assuming Von for the driving voltage at the point P1 and Voff for the driving voltage at the point P2, the variable capacitor F1 may be used as a capacitive switch that switches a capacitance between at the driving voltage Voff and at the driving voltage Von.
However, when the variable capacitor F1 is actually used as a capacitive switch, the driving voltage (direct current voltage) of the movable electrode 104 should be only applied to that movable electrode 104 and not applied to other circuits connected to the variable capacitor F1. Therefore, it is necessary to provide a circuit (hereinafter, the circuit is referred to as “DC block”) for blocking the driving voltage of the movable electrode 104.
FIG. 31 shows an equivalent circuit diagram of the variable capacitor F1 used as a capacitive switch connected in parallel with a signal line for an unbalanced alternating current signal.
As shown in FIG. 31, the fixed electrode 102 and the movable electrode 104 are respectively connected to a ground and a signal line 110, for example, through which an RF signal (alternating current signal) flows, and a driving voltage is applied from a DC power 113 to the movable electrode 104. A capacitor 111 as a DC block is provided between the signal line 110 and the variable capacitor F1 in order not to apply a driving voltage from the DC power 113 to the signal line 110. In addition, an inductor 114 as a circuit (hereinafter, the circuit is referred to as “RF block”) for blocking an RF signal, is provided between the DC power 113 and the movable electrode 104. The inductor can prevent an RF signal flowing through the signal line 110 from being bypassed to a ground in a path between the capacitor 111 and the DC power 113.
The capacitance of the capacitor 111 must be sufficiently large with respect to the variable capacitor F1 not to influence the characteristic of the variable capacitor F1. Therefore the size of a capacitor 111 is large for a large capacitance. Thus, there has been a limit on miniaturization of a device that employs the variable capacitor F1.
In addition, the driving voltage Voff needs to be a voltage larger than a voltage induced between the fixed electrode 102 and the movable electrode 104 by an RF signal flowing through the signal line 110. Therefore, it is necessary to set the driving voltage Voff to be large when a large RF signal flows in the signal line 110. As is apparent from the characteristic shown in FIG. 30, the driving voltage Von is set to be larger than the driving voltage Voff. Consequently the driving voltage Von is so large during the maximum capacitance that an electrification phenomenon of the dielectric layer 105 is more likely to occur.